Structures and phase transitions

This was the first of six high-pressure sessions at the Glasgow Congress. S. Hull (ISIS Facility, UK) opened the proceedings with a lucid account of his high-pressure high-temperature studies of binary halides. These simple materials exhibit surprisingly complex behaviour, with transitions from cubic NaCl to CsCl structures progressing through a number of other lower-symmetry intermediates. The complex behaviour exhibited by simple materials was reemphasised by U. Schwarz (MPI, Dresden, Germany), who described recent diffraction studies of Cs, Si and Rb. These reveal that the long-uncertain structures of Cs-V, Si-VI and Rb-VI are orthorhombic with a new structure type. However, although the crystallographic structures are the same in each case, the calculated electron densities are different. The next three invited talks focused on molecular systems. N. Hamaya (Ochanomizu U., Japan) described studies of AX₄ structures at SPring-8. A new re-crystallised phase of SnI₄ existing at 7GPa and 500K was studied using anomalous dispersion techniques to determine the structure. The results were consistent with a disordered cubic structure. An amorphous phase of GeI₄ was also reported above 30GPa. The breakdown in the molecular structure of CO2 at high-pressure to form a polymer was described by C-S. Yoo (Lawrence Livermore Nat. Lab., USA). This polymeric form, which may act as a frequency-doubler, has a bulk modulus very similar to that of cubic- BN. Unfortunately, it is not recoverable at ambient conditions. The ability of pressure to create new materials was the subject of a talk by S. Tsuneyuki (Tokyo U., Japan) who described calculations aimed at identifying new high-pressure structures. Calculations of a new phase of SnI4 above 61GPa described by Hamaya suggest it is a substitutional solid solution of Sn and atomic iodine. In the final talk of the session, J.B. Parise (SUNY, StonyBrook, USA) reviewed the large number of current (and conflicting) diffraction, transport and magnetic studies of FeS. A clear picture is beginning to emerge from conflicting results that probably arise from slight changes in sample homogeneity and composition. The session included three short oral summaries of poster presentations on studies of ice-VI and ice-VII using single-crystal neutron diffraction (M. Guthrie, Edinburgh U., UK); complex structures of lanthanide (La-Pr-Ce) alloys (O. Degtyareva, Paderborn U., Germany); and calculations of the high-pressure phases of PON, which is isoelectronic with SiO2 (D. Klug, NRC Ottawa, Canada).

M. I. McMahon, Co-chair